U.S. Department of Transportation
Federal Highway Administration
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Washington, DC 20590
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The introduction to this issue brief provides an overview of traffic signals (purpose, warrants for signal installation, advantages, disadvantages, and factors to consider) followed by an introduction to the contents of this issue brief (crash reduction factors, presentation of the crash reduction factors, and using the tables).
Traffic signals are used to assign vehicular and pedestrian right-of-way. They are used to promote the orderly movement of vehicular and pedestrian traffic and to prevent excessive delay to traffic.
Traffic signals should not be installed unless one of the warrants specified by the Manual on Uniform Traffic Control Devices (MUTCD) has been satisfied. The satisfaction of a warrant is not in itself justification for a signal. A traffic engineering study must be conducted to determine whether the traffic signal should be installed. The installation of a traffic signal requires sound engineering judgment, and must balance the following, sometimes conflicting, goals:
The MUTCD lists eight warrants for the placement of traffic signals. Readers are encouraged to review Part 4 of the MUTCD for more specific information regarding signal warrants. Access management considerations and the spacing of signals on arterial roadways are critical elements of system efficiency and operational safety. The basic question that must be answered is, "Will this intersection operate better with or without a traffic signal?"
Traffic signals that are properly located and operated are likely to:
Traffic control signals are often considered a panacea for all traffic problems at intersections. This belief has led to the installation of traffic control signals at many locations where they are not needed and where they may adversely affect the safety and efficiency of vehicular, bicycle, and pedestrian traffic.
Even when justified by traffic and roadway conditions, traffic control signals can be ill designed, ineffectively placed, improperly operated, or poorly maintained. Unjustified or improper traffic control signals can result in one or more of the following disadvantages:
As angle crashes tend to be more severe than rear-end crashes, traffic engineers are usually willing to trade off an increase in the number of rear-end crashes for a decrease in the number of angle crashes, but if an intersection does not have an angle-crash problem, the tradeoff does not apply, and the installation of traffic signals can actually cause a deterioration in the overall safety at the intersection.
A number of factors should be considered when planning to signalize an intersection. These factors include:
The following changes may decrease crashes:
This issue brief documents estimates of the crash reduction that might be expected if a specific countermeasure or group of countermeasures is implemented with respect to traffic signals. The crash reduction estimates are presented as crash reduction factors (CRFs).
Traffic engineers and other transportation professionals can use the information contained in this issue brief when asking the following types of question: Which countermeasures might be considered at the signalized intersection of Maple and Elm streets, an intersection that is experiencing a high number of crashes? What changes in the number of crashes are possible with the various countermeasures?
A CRF is the percentage crash reduction that might be expected after implementing a given countermeasure. In some cases, the CRF is negative (i.e., the implementation of a countermeasure is expected to lead to a percentage increase in crashes).
One CRF estimate is provided for each countermeasure. Where multiple CRF estimates were available from the literature, selection criteria were used to choose which CRFs to include in the issue brief:
Where these criteria could not be met, a CRF may still be provided. In these cases, it is recognized that the reliability of the estimate of the CRF is low, but the estimate is the best available at this time. The CRFs in this issue brief may be periodically updated as new information becomes available.
The Desktop Reference for Countermeasures lists all of the CRFs included in this issue brief and adds many other CRFs available in the literature. A few CRFs found in the literature were not included in the Desktop Reference. These CRFs were considered to have too large a range or too large a standard error to be meaningful, or the original research did not provide sufficient detail for the CRF to be useful.
A CRF should be regarded as a generic estimate of the effectiveness of a countermeasure. The estimate is a useful guide, but it remains necessary to apply engineering judgment and to consider site-specific environmental, traffic volume, traffic mix, geometric, and operational conditions that will affect the safety impact of a countermeasure. The user must ensure that a countermeasure applies to the particular conditions being considered. The reader is also encouraged to obtain and review the original source documents for more detailed information and to search databases such as the National Transportation Library (http://ntlsearch.bts.gov) for information that becomes available after the publication of this issue brief.
In the Table presented in this issue brief, the crash reduction estimates are provided in the following format:
The CRF is the value selected from the literature.
The use of the color blue and the italicizing of words used in the text (except for words associated with a specific document) are associated with new information provided by the Highway Safety Manual, April 2009 draft, as listed in Reference 21 at the end of this issue brief.
The standard error is given where available. The standard error is the standard deviation of the error in the estimate of the CRF. The true value of the CRF is unknown. The standard error provides a measure of the accuracy of estimate of the true value of the CRF. The August 2008 edition of Issue Brief 5 used the phrase "relatively small" to indicate that a CRF is "relatively accurately known." Relatively small was not explicitly defined several years ago; however, its intention is congruent with the definition used in this edition of the Issue Brief: relatively small is defined as a CRF with a standard error ≤ 10. This is equivalent to the Highway Safety Manual AMFs (Accident Modification Factors) with standard errors of ≤ 0.10.
A "relatively large" standard error associated with a CRF is defined as >10 and indicates that the CRF is "not accurately known." The standard error may be used to estimate a confidence interval of the true value of the CRF. (An example of a confidence interval calculation is given below.) The REF is the reference number for the source information. As an example, the CRF for the countermeasure "provide protected left-turn phase for left-turn fatal/injury crashes" is:
The following points should be noted:
The CRFs for traffic signal-related crashes are presented in the Signalization Countermeasures Table that summarizes the available information.
Readers familiar with the previous editions of this issue brief will notice the following changes:
Table 1, Signalization Countermeasures is divided into three sections: signal operations countermeasures; signal hardware countermeasures; and combination signal and other countermeasures. This table is also found in Issue Brief No. 8, which includes a more comprehensive toolbox of countermeasures for consideration at intersections.
The following points should be noted:
CRF is a crash reduction factor, which is an estimate of the percentage reduction that might be expected after implementing a given countermeasure. A number in bold indicates a rigorous study methodology and a small standard error (≤10) in the value of the CRF. Standard error, where available, is the standard deviation of the error in the estimate of the CRF.
REF is the reference number for the source information.
Additional crash types identified in the Other Crashes column:
l: Emergency vehicle
|Countermeasure||Crash Severity||Control||Area Type||Configuration||All Crashes||Left-Turn Crashes||Rt-Angle Crashes||Rear-end Crashes||Sideswipe Crashes||Other Crashes||Major/ Minor Daily Traffic Volume (veh/day)|
|Signal Operations Countermeasures|
|Add all-red clearance interval (from 0 to 1 second)||All||Signal||Urban|
|Add exclusive pedestrian phasing||All||Signal||0(44)14||k (34)7|
|Convert exclusive leading protected to exclusive lagging protected||All||Signal||-15(19)6||-49(54)6|
|Convert permissive or permissive/ protected to protected only left-turn phasing||All||9920|
|Convert permissive to permissive/ protected left-turn phasingAll||All||1620|
|Convert protected left-turn phase to protected/ permissive||All||Signal||-20(17)15||-65(71)6||4(22)6|
|Convert protected left-turn phase to protected/ permissive||Fatal/ Injury||Signal||-10(25)15|
|Convert permissive to protected||All||Signal||Urban||4-leg or 3-leg||6(10)21||99(1)21|
|Convert permissive to protected/ permissive or permissive/ protected phasing||Injury||Signal||Urban||4-leg||16(2)21||3,000- 77,000/ 10-45,500|
|Convert permissive to protected/ permissive or permissive/ protected phasing||All||Signal||Urban||4-leg||121|
|Convert permissive to protected left-turn phase on multiple approaches||All||Signal||on 1 approach||621|
|Convert permissive to protected left-turn phase on multiple approaches||All||Signal||on 2 approaches||1121|
|Convert permissive to protected left-turn phase on multiple approaches||All||Signal||on 3 approaches||1721|
|Convert permissive to protected left-turn phase on multiple approaches||All||Signal||on 4 approaches||2221|
|Convert permissive to protected/ permissive or permissive/ protected left turn phase on multiple approaches||All||Signal||on 1 approach||121|
|Convert permissive to protected/ permissive or permissive/ protected left turn phase on multiple approaches||All||Signal||on 2 approaches||221|
|Convert permissive to protected/ permissive or permissive/ protected left turn phase on multiple approaches||All||Signal||on 3 approaches||321|
|All||Signal||on 4 approaches||421|
|Convert protected/ permissive left-turn phase to permissive/ protected||All||Signal||-13(19)8||33(22)8|
|Improve signal timing [to intervals specified by the ITE Determining Vehicle Change Intervals: A Proposed Recommended Practice (1985)]||All||Signal||4-leg||8(9)15||4(18)15||-12(16)15||h 4212|
|Improve signal timing [to intervals specified by the ITE Determining Vehicle Change Intervals: A Proposed Recommended Practice (1985)]||All||Signal||All||f 55|
|Improve signal timing [to intervals specified by the ITE Determining Vehicle Change Intervals: A Proposed Recommended Practice (1985)]||All||Signal||754|
|Fatal/ Injury||Signal||554||304||a 754|
|Fatal/ Injury||Signal||b 624|
|Fatal/ Injury||Signal||All||f 95|
|Fatal/ Injury||k 3715|
|Increase yellow change interval||All||Signal||154||304|
|Install emergency vehicle pre-emption systems||All||l 7016|
|Modify signal phasing (implement a leading pedestrian interval)||All||Signal||k 57|
|Provide Advanced Dilemma Zone Detection for rural high-speed approaches||Fatal/ Injury||Signal||Rural||4-leg (1 app)||3919|
|Provide protected left-turn phase||Fatal/ Injury||Signal||Urban||17(4)21||25(2)21|
|Provide protected left-turn phase||All||Signal||304||414||544||274||c 274||<5,000/ lane (Total)|
|Provide protected left-turn phase||All||Signal||364||464||564||354||c 354||<5,000/ lane (Total)|
|Provide protected left-turn phase||All||Signal||274||484||634||314||c 314|
|Provide protected/ permissive left-turn phase (leading green arrow)||Fatal/ Injury||Signal||Urban||17(2)9||25(2)9|
|Provide signal coordination||All||Signal||327|
|Provide split phases||All||Signal||257|
|Remove flash mode (late night/ early morning)||All||Signal||297||75(19)7|
|Replace existing WALK/ DON'T WALK signals with pedestrian countdown signal heads||All||Signal||Urban||k 2510|
|Signal Hardware Countermeasures|
|Add 3-inch yellow retroreflective sheeting to signal backplates||All||Signal||Urban||15(51)17|
|Add additional signal and upgrade to 12-inch lenses||All||Signal||4-leg||h 3112|
|Add additional signal and upgrade to 12-inch lenses||All||Signal||4-leg||h 1712|
|Add signal (additional primary head)||All||Signal||Urban||4-leg||282||352||282|
|Add signal (additional primary head)||Fatal/ Injury||Signal||Urban||4-leg||172|
|Add signal (additional primary head)||PDO||Signal||Urban||4-leg||312|
|Convert signal from pedestal-mounted to mast arm||All||Signal||4916||1216||7416||4116|
|Convert signal from pedestal-mounted to mast arm||Fatal/ Injury||Signal||4416|
|Convert signal from pedestal-mounted to mast arm||PDO||Signal||5116|
|Improve visibility of signal heads (increase signal lens size, install new backboards, add reflective tape to existing backboards, and/or install additional signal heads)||All||Signal||Urban||718||d 618|
|Improve visibility of signal heads (increase signal lens size, install new backboards, add reflective tape to existing backboards, and/or install additional signal heads)||All||Signal||Urban||e 618|
|Improve visibility of signal heads (increase signal lens size, install new backboards, add reflective tape to existing backboards, and/or install additional signal heads)||Fatal/ Injury||Signal||Urban||318|
|Improve visibility of signal heads (install two red displays in each head)||All||Signal||97||367|
|Install larger signal lenses (12 inch)||All||Signal||117||4614|
|Install larger signal lenses (12 inch)||All||Signal||Urban||2417|
|Install larger signal lenses (12 inch)||Fatal/ Injury||Signal||Urban||1617|
|Install signal backplates only||All||Signal||137||507|
|Install signal backplates (or visors)||All||Signal||204|
|Install signals||All||No Signal||337||3813||j 5013|
|Install signals||All||No Signal||384||749||229||c 224||<5,000/ lane (Total)|
|Install signals||All||No Signal||204||439||209||c 204||>5,000/ lane (Total)|
|Install signals||All||No Signal||Rural||1513|
|Install signals||All||Stop||Rural||3-leg or 4-leg||44(3)21||60(6)21||77(2)21||-58(20)21||3,300-30,000/ 100-10,300|
|Install signals||Fatal||No Signal||3813|
|Install signals||Fatal/ Injury||Stop||Urban||3-leg||14(32)11||34(45)11||-50(51)11||11,750-42,000/ 900-4000|
|Install signals||Fatal/ Injury||Stop||Urban||4-leg||23(22)11||67(20)11||-38(39)11||12,650- 22,400/ 2,400- 3,625|
|Install signals||PDO||No Signal||-1513|
|Install signals (temporary)||Fatal/ Injury||No Signal||394||504|
|Install signals (temporary)||PDO||No Signal||114||734||a 834|
|Install signals (to have one over each approach lane)||All||All||463|
|Remove unwarranted signals||All||Signal||Urban||24(9)21||24(10)21||29(20)21||d 305|
|Remove unwarranted signals||All||Signal||Urban||e 225|
|Remove unwarranted signals||All||Signal||Urban||g 315|
|Remove unwarranted signals||Fatal/ Injury||Signal||Urban||535|
|Remove unwarranted signals||PDO||Signal||Urban||245|
|Remove unwarranted signals||Pedestrian||Signal||Urban||One-lane one-way streets excluding major arterials||18(30)21|
|Replace signal lenses with optical lenses||All||Signal||177||104||104||104||a 204|
|Combination Signal and Other Countermeasures|
|Install left turn lane and add turn phase||All||Signal||587|
|Install signals and add channelization||Fatal/ Injury||No Signal||674||544||b 354|
|Install signals and add channelization||PDO||No Signal||244||634||a 274|
|Note: Any CRF with a reference of 21 is added to this version of the Intersection Safety Issue Brief 5.|
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